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Big Roof Britain Solar Payback Index Standing Charge Drag

Solar Panels in TW15 (Spelthorne)

See if solar pays off for you in Spelthorne — price, savings and payback in under a minute (official UK data).

Solar Calculator

Change the settings below to see how costs and savings work for your situation.

System Size

Roof Direction

Battery Storage

Export Value

Your Numbers

Break-even
12.2 years
After this, it's pure profit
Upfront Cost
£10,173
What you pay to get started
Annual Benefit
£831
What you save every year
Profit Over 25 Years
£10,591
After paying itself off, you're this much ahead

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How we calculate this

Solar generation: Based on typical south-facing roof at 35° angle. We account for weather, dirt, and system losses (14%).

How much you'll use directly:

  • Without battery: You'll use about 40% of what you generate. The rest gets sold back to the grid.
  • With 5 kWh battery: You'll use about 70% (battery stores extra for evening use).
  • With 10 kWh battery: You'll use about 75% (more storage means less exported).
  • Panel-only payback: Bigger systems don’t shorten payback—cost and savings rise together per kW.
  • Battery & payback: Batteries raise self-use and cut bills, but added hardware cost often lengthens break-even at typical UK rates.
  • When a battery can still make sense:
    • Evening-heavy usage patterns
    • Time-of-use tariffs / cheap night rates
    • Low export rate vs. high unit rate
    • Heat pump or EV charging synergy
    • Optional backup/resilience (system-dependent)
    • Lower evening-hour CO₂ vs. grid

Costs: Battery prices are typically £700 per kWh. Export tariffs vary by supplier (usually 5–15p per kWh).

Grid connection: Systems up to about 3.7 kW typically need just a notification (G98). Larger systems may need permission (G99).

These are typical values based on average usage patterns. Your actual results will vary based on your roof, shading, and how you use electricity.

Your area in context

In Southern, the current unit rate is 26.48p/kWh with a 44.62p/day standing charge. Your postcode’s south-facing sunshine yield is 1001 kWh/kW — 0 below the regional median (1001).

Read the full story

A typical roof here lands around 5.0 kW, with most eligible homes comfortably fitting within 3–6 kW. Each kilowatt of panels produces roughly 1001 kWh per year on a south-facing roof. We value self-used electricity at 26.5p and exported surplus at a baseline 10p via SEG. Use the calculator above for your exact tariff.

Unit rate
26.5p/kWh
Standing charge
44.6p/day
Sunshine yield (south)
1001 kWh/kW
Export price (baseline)
10p/kWh

Browse the scenarios below. Each battery size has its own expand — open the one that matches your plan (or just explore). For each system size (2–8 kW) we show annual output, how much you use directly, what gets exported, annual benefit, upfront cost, break-even, and a 20-year net. Negative numbers are possible — that’s the honest trade-off when adding more battery than your usage profile justifies.

No battery 5 kW example → Annual benefit: £831, Upfront: £10,173, Break-even: 12.2 yrs
System Annual output Self-use Exported Bill saving Export income Annual benefit Upfront cost Break-even 20-year net
2 kW 2002 kWh 801 kWh 1201 kWh £212 £120 £332 £4,069 12.2 yrs £2,575
3 kW 3003 kWh 1201 kWh 1802 kWh £318 £180 £498 £6,104 12.2 yrs £3,863
4 kW 4005 kWh 1602 kWh 2403 kWh £424 £240 £664 £8,138 12.2 yrs £5,151
5 kW 5006 kWh 2002 kWh 3003 kWh £530 £300 £831 £10,173 12.2 yrs £6,438
6 kW 6007 kWh 2403 kWh 3604 kWh £636 £360 £997 £12,207 12.2 yrs £7,726
7 kW 7008 kWh 2803 kWh 4205 kWh £742 £420 £1,163 £14,242 12.2 yrs £9,013
8 kW 8009 kWh 3204 kWh 4805 kWh £848 £481 £1,329 £16,276 12.2 yrs £10,301

Assumptions: south-facing, sensible tilt; self-use shares from site rules; export at 10p baseline. Costs: panels £/kW = 2,035, battery £/kWh = 700.

3 kWh battery 5 kW example → Annual benefit: £996, Upfront: £12,273, Break-even: 12.3 yrs
System Annual output Self-use Exported Bill saving Export income Annual benefit Upfront cost Break-even 20-year net
2 kW 2002 kWh 1201 kWh 801 kWh £318 £80 £398 £6,169 15.5 yrs £1,795
3 kW 3003 kWh 1802 kWh 1201 kWh £477 £120 £597 £8,204 13.7 yrs £3,743
4 kW 4005 kWh 2403 kWh 1602 kWh £636 £160 £796 £10,238 12.9 yrs £5,690
5 kW 5006 kWh 3003 kWh 2002 kWh £795 £200 £996 £12,273 12.3 yrs £7,638
6 kW 6007 kWh 3604 kWh 2403 kWh £954 £240 £1,195 £14,307 12.0 yrs £9,586
7 kW 7008 kWh 4205 kWh 2803 kWh £1,113 £280 £1,394 £16,342 11.7 yrs £11,533
8 kW 8009 kWh 4805 kWh 3204 kWh £1,272 £320 £1,593 £18,376 11.5 yrs £13,481

Assumptions: south-facing, sensible tilt; self-use shares from site rules; export at 10p baseline. Costs: panels £/kW = 2,035, battery £/kWh = 700.

5 kWh battery 5 kW example → Annual benefit: £1,078, Upfront: £13,673, Break-even: 12.7 yrs
System Annual output Self-use Exported Bill saving Export income Annual benefit Upfront cost Break-even 20-year net
2 kW 2002 kWh 1402 kWh 601 kWh £371 £60 £431 £7,569 17.6 yrs £1,055
3 kW 3003 kWh 2102 kWh 901 kWh £557 £90 £647 £9,604 14.8 yrs £3,333
4 kW 4005 kWh 2803 kWh 1201 kWh £742 £120 £862 £11,638 13.5 yrs £5,610
5 kW 5006 kWh 3504 kWh 1502 kWh £928 £150 £1,078 £13,673 12.7 yrs £7,888
6 kW 6007 kWh 4205 kWh 1802 kWh £1,113 £180 £1,294 £15,707 12.1 yrs £10,165
7 kW 7008 kWh 4906 kWh 2102 kWh £1,299 £210 £1,509 £17,742 11.8 yrs £12,443
8 kW 8009 kWh 5606 kWh 2403 kWh £1,485 £240 £1,725 £19,776 11.5 yrs £14,720

Assumptions: south-facing, sensible tilt; self-use shares from site rules; export at 10p baseline. Costs: panels £/kW = 2,035, battery £/kWh = 700.

7.5 kWh battery 5 kW example → Annual benefit: £1,103, Upfront: £15,423, Break-even: 14.0 yrs
System Annual output Self-use Exported Bill saving Export income Annual benefit Upfront cost Break-even 20-year net
2 kW 2002 kWh 1462 kWh 541 kWh £387 £54 £441 £9,319 21.1 yrs £-497
3 kW 3003 kWh 2192 kWh 811 kWh £581 £81 £662 £11,354 17.2 yrs £1,880
4 kW 4005 kWh 2923 kWh 1081 kWh £774 £108 £882 £13,388 15.2 yrs £4,256
5 kW 5006 kWh 3654 kWh 1352 kWh £968 £135 £1,103 £15,423 14.0 yrs £6,633
6 kW 6007 kWh 4385 kWh 1622 kWh £1,161 £162 £1,323 £17,457 13.2 yrs £9,009
7 kW 7008 kWh 5116 kWh 1892 kWh £1,355 £189 £1,544 £19,492 12.6 yrs £11,386
8 kW 8009 kWh 5847 kWh 2162 kWh £1,548 £216 £1,764 £21,526 12.2 yrs £13,762

Assumptions: south-facing, sensible tilt; self-use shares from site rules; export at 10p baseline. Costs: panels £/kW = 2,035, battery £/kWh = 700.

10 kWh battery 5 kW example → Annual benefit: £1,119, Upfront: £17,173, Break-even: 15.3 yrs
System Annual output Self-use Exported Bill saving Export income Annual benefit Upfront cost Break-even 20-year net
2 kW 2002 kWh 1502 kWh 501 kWh £398 £50 £448 £11,069 24.7 yrs £-2,115
3 kW 3003 kWh 2253 kWh 751 kWh £596 £75 £672 £13,104 19.5 yrs £328
4 kW 4005 kWh 3003 kWh 1001 kWh £795 £100 £895 £15,138 16.9 yrs £2,770
5 kW 5006 kWh 3754 kWh 1251 kWh £994 £125 £1,119 £17,173 15.3 yrs £5,213
6 kW 6007 kWh 4505 kWh 1502 kWh £1,193 £150 £1,343 £19,207 14.3 yrs £7,655
7 kW 7008 kWh 5256 kWh 1752 kWh £1,392 £175 £1,567 £21,242 13.6 yrs £10,098
8 kW 8009 kWh 6007 kWh 2002 kWh £1,591 £200 £1,791 £23,276 13.0 yrs £12,540

Assumptions: south-facing, sensible tilt; self-use shares from site rules; export at 10p baseline. Costs: panels £/kW = 2,035, battery £/kWh = 700.

13.5 kWh battery 5 kW example → Annual benefit: £1,136, Upfront: £19,623, Break-even: 17.3 yrs
System Annual output Self-use Exported Bill saving Export income Annual benefit Upfront cost Break-even 20-year net
2 kW 2002 kWh 1542 kWh 461 kWh £408 £46 £454 £13,519 29.8 yrs £-4,433
3 kW 3003 kWh 2313 kWh 691 kWh £612 £69 £681 £15,554 22.8 yrs £-1,924
4 kW 4005 kWh 3083 kWh 921 kWh £817 £92 £909 £17,588 19.4 yrs £584
5 kW 5006 kWh 3854 kWh 1151 kWh £1,021 £115 £1,136 £19,623 17.3 yrs £3,093
6 kW 6007 kWh 4625 kWh 1382 kWh £1,225 £138 £1,363 £21,657 15.9 yrs £5,601
7 kW 7008 kWh 5396 kWh 1612 kWh £1,429 £161 £1,590 £23,692 14.9 yrs £8,110
8 kW 8009 kWh 6167 kWh 1842 kWh £1,633 £184 £1,817 £25,726 14.2 yrs £10,618

Assumptions: south-facing, sensible tilt; self-use shares from site rules; export at 10p baseline. Costs: panels £/kW = 2,035, battery £/kWh = 700.

Figures are illustrative and use the same method as the calculator above. For exact results, adjust your roof direction, tariff, and usage in the calculator.

Insights for TW15

Figures below are tailored to TW15 — click through for full league tables.

Big-Roof Britain Rank #1404 of 2182
4.7 kWp (UK avg 5.1 kWp)
~10 panels (≈430 W)
Solar Payback Index Rank #890 of 2997
10.7 years (UK avg 11.3 years)
~£954 /yr • £10,173 upfront
Standing-Charge Drag Rank #744 of 2997
17.1% (UK avg 21.7%) drag on savings
£163 fixed • ~£954 /yr value

Your Area at a Glance

Your Electricity Rate
26.48p/kWh
Daily Fixed Charge
44.62p/day
Local Sunshine
1001kWh/kW
A 1 kW system generates ~1001 kWh per year. A typical home uses ~2,700 kWh per year.
Typical System Size
5.0kW
For houses & bungalows in this area

Local Property Data

Property Types

Houses: 50% • Flats: 34% • Bungalows: 10% Based on 7,548 properties

Coverage

Spelthorne (99%)|Hounslow (1%)

Areas Covered

Ashford Common, Ashford Town, Ashford East, Ashford North and Stanwell South, Laleham and Shepperton Green, Staines South, Feltham West

Data Sources & Assumptions

Solar yield: PVGIS (EU Joint Research Centre), January 2025 data

Electricity rates: Ofgem price cap, Q4 2025

Installation costs: MCS certified installer data, March 2025

Property data: EPC open data (Open Government Licence), aggregated

All estimates are typical values. Actual results vary based on your specific situation, roof characteristics, and usage patterns.

Compare nearby areas

Nearby in Southern

TW16 (Spelthorne)
≈2.5 km away
Local sunshine: ~1,001 kWh/kWp · -0% vs TW15
TW13 (Hounslow)
≈3.2 km away
Local sunshine: ~1,001 kWh/kWp · +0% vs TW15
TW19 (Spelthorne)
≈3.5 km away
Local sunshine: ~1,001 kWh/kWp · -0% vs TW15
TW14 (Hounslow)
≈3.6 km away
Local sunshine: ~1,001 kWh/kWp · -0% vs TW15
TW18 (Spelthorne)
≈4.0 km away
Local sunshine: ~998 kWh/kWp · -0% vs TW15
TW6 (Hillingdon)
≈4.6 km away
Local sunshine: ~1,007 kWh/kWp · +1% vs TW15

Similar sunshine across the UK

PE2 (Peterborough)
~1,001 kWh/kWp · -0% vs TW15
PE3 (Peterborough)
~1,001 kWh/kWp · +0% vs TW15
PE1 (Peterborough)
~1,001 kWh/kWp · +0% vs TW15